Assessing water quality in Sultanate of Oman using a newly developed dynamic speciation technique

Al Kasbi, Mohammed (2016) Assessing water quality in Sultanate of Oman using a newly developed dynamic speciation technique. PhD thesis, UNSPECIFIED.

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Rapid increases in coastal developments in the Sultanate of Oman recently, including large ports with industries, crude oil refineries, mining and industrial activities, have often produced point sources of metal and organic contaminations in the local marine systems. Although advances have been made in measuring and understanding the trace metal speciation in aquatic system using a dynamic technique DGT (Diffusive Gradients in Thin-films) or using other equilibrium techniques, measuring metal speciation in situ in seawater that polluted by crude oil is still extremely challenging. Without the technology and the information, it is difficult to assess the effects of oil ligands on metal speciation associated with the oil refineries and industries. In the present study, the DGT technique has been developed further for measuring metal speciation in seawaters in the presence of oil. The influence of oil ligands on the speciation of trace metals (Cd, Co, Cu, Ni, Pb and Zn) has been investigated in synthetic solutions of 0.5 and 0.7 mol/L NaCl containing different level of Oman crude oil. DGT of different types in diffusive layers (pore size and thicknesses), binding layers (Chelex resin and Fe oxide) and different types of membranes (dialysis membranes (1000 MWCO and 3500 MWCO) and 0.05 mm thickness Nafion112 membrane) were used. The diffusion coefficients were determined for each metal, using a diffusion cell, through filter membrane and diffusive gel with and without dialysis and Nafion112 membranes. The measured diffusion coefficients of all metals in diffusive gel plus dialysis membrane were significantly lower than gel alone. When Nafion112 membrane wasused with the diffusive gel, the diffusion coefficients of all metals were much lower,up to 80 times lower for Cu, Cd and Ni. The potential of using DGT for determining labile metals species in seawater in the presence of crude oil was evaluated in mixed solution of NaCl and oil with varying mixing periods (0.25 day, 0.5 day, 1 day, 2 days, 4 days and 6 days) and oil concentrations (1, 3, 5, 7, 25 OWR% (oil water ratio), w/v). It was demonstrated that organic ligands from crude oil influenced the lability of trace metals. The combination of five types of DGT devices (OP-DGT, RG-DGT, (dialysis 1000Da)-DG, (dialysis 3500Da)-DGT, (Nafion 112)-DGT) were used for speciation of metals at 1 and 4 oil water ratio (OWR%), and 48 h mixing time. Lower concentrations of labile metals were determined with the higher level of oil in the synthetic solution when using Nafion 112-DGT, due to the exclusion of large colloids and negatively charged metal complexes by Nafion 112 membrane. No significance differences were observed between OP-DGT and RG-DGT for Co, Ni, Zn and Pb at 1% OWR and Co, Ni and Pb in at 4% OWR. This suggests the labile complexes of metals and organic ligands from oil may be smaller than the pore size of the restricted diffusive gel. Comparing (Dialysis 1000MWCO)-DGT and 1000MWCO- Micro Float microdialysis measurements indicated that Co, Cd, Cu and Zn were complexed with ligands small enough to pass through the 1000 MWCO- Micro Float microdialysis but not labile enough to be retained in the (Dialysis 1000 MWCO)-DGT samplers. DGT technique was applied to soils contaminated with both metals and crude oil to evaluate the effect of oil level and aging on the availability of metals in soils. The dissolved metals in soil solution (Csoln), labile concentrations by DGT (CDGT) and extractable concentration by CaCl2 were measured. Values of labile pool size, Kd, and R (ratio of CDGT and Csoln) were calculated and the kinetic parameters (response time Tc and desorption rate k−1) were obtained using DIFS (DGT Induced Fluxes in Soils and sediments) model. Increases of crude oil treatments in soils promoted the formation of non-labile species of Cu and Pb and limited the availability of both metals. The time scales of the kinetic exchanges obtained from DGT measurements and DIFS modelling was reflected by the range of determined Tc values: from seconds to 3 hours. The gradual decrease of Kd values for Cu with time in three studied soils with and without oil application may be attributed to the strong tendency of Cu to be associated with the solid phase with increasing incubation time. The Kd values of Ni were generally decreased with adding of crude oil amendments in three soils may be due to the release of Ni species from solid phase and crude oil to soil solution. Three soils were resupply of Cd and Pb in general very slow to measure. The generally slower release rates of Cd and Pb compared to other studied metals may reflect the lower concentration of Cd and Pb which allowed a higher proportion of them to associate strongly with organic ligands sites with slower release rates. The trend changes for k-1 values of studied metals in three selected soils were influenced by crude oil amendments and aging time. All developed DGT of different types, including DGT with ferrihydrite for oxyanions (As, Se, Mo, V, Sb and W), were deployed in situ in seawater and groundwater around refineries and industrial areas in Oman. Soils and sediments in those areas were also collected for DGT deployments. The labile concentrations of Cr, Co, Ni, Cu, Cd, and Pb were generally low and not significantly different at all the sampling locations due to the formation of strong metals complexes with organic ligands that are less labile. The measured concentrations of labile V in all selected locations were much lower than the total V concentration in the filtered grab samples (CSol (0.45 μm filtered)) due to high levels of large colloidal ligands from crude oil. The low proportion of Ni labile species (11%-29%) was measured by (Nafion112)-DGT in all selected open wells because Ni is mainly complexed by negatively charged ligands and they were excluded by Nafion112 membrane. The labile Fe, Ni and Mn concentrations in Wells-1 and -2 were much higher than those in Well-3 and (RIE)’swells reflecting the occurrence of more intense oil and industrial contamination in the area. For coastal sediments near the refinery discharge point at Al Fahal port, the DGT measured labile concentrations of Fe, Ni, Cu, and V were higher near the discharge point. The DGT concentrations of Al, Fe and Mn in soils collected from SIR ranged from (12-22 μg/L), (14-63 μg/L) and (9-17μg/L) respectively, in line with the background level of those metals. For 15 selected soils, their Kd ranged from (1.8-33) ml/g for (Al, Cr, Mn, Fe, Ni and Cu). The Kd values of Pb were higher at soil locations (L2, L7, L8 and L11) of 266, 65, 43 and 55 gm/L respectively due to the higher metal retention by the solid phase through sorption reactions and low concentration in the soil solution. The obtained results suggest that risk assessment of sandy soils could be carried out using measurements of metals in soil solutions. However, devices such as DGT dynamic technique, which respond to the kinetics of supply, are essential to evaluate available metal in high pH and crude oil contaminated soils. Because DGT is a dynamic technique and it can be readily deployed in situ, in seawaters, groundwaters, soils, and sediments, it provides kinetic information directly in crude oil contaminated environmental systems and allows better understanding and risk assessments.

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Thesis (PhD)
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01 Jul 2016 09:28
Last Modified:
12 Sep 2023 00:18